Note that prerendering browsing context are never auxiliary

Closes #73.

prerendering.bs

@@ -203,6 +203,8 @@ Every [=navigable=] has a <dfn for="navigable">loading mode</dfn>, which is one
 
 By default, a [=navigable=]'s [=navigable/loading mode=] is "`default`". A navigable whose [=navigable/loading mode=] is "`prerender`" is known as a <dfn>prerendering navigable</dfn>. A [=prerendering navigable=] that is also a [=top-level traversable=] is known as a <dfn>prerendering traversable</dfn>.
 
+<p class="note">[=Prerendering navigables=]'s [=navigable/active browsing contexts=] are never [=auxiliary browsing contexts=].
+
 <p class="note">Although there are only two values for the [=navigable/loading mode=], we use a flexible structure in anticipation of other future loading modes, such as those provided by fenced frames, portals, and uncredentialed (cross-site) prerendering. It's not yet clear whether that anticipation is correct; if, as those features gain full specifications, it turns out not to be, we will instead convert this into a boolean.
 
 Every [=prerendering traversable=] has a <dfn for="prerendering traversable">prerender initial response search variance</dfn>, which is a [=URL search variance=] or null, and is initially null.